Look into these websites:

http://creativecommons.org/

http://www.opengamingfoundation.org/ogl.html

System Legal:

Any systemic elements may be used freely in either profit or not-for-profit work. Also any.

Setting Legal:

Contact the author before using any setting elements in for-profit work. Any non-profit work (either

Copying, re-posting, and money – don’t sell this stuff unless you’ve obtained a license from me. Copying for personal use is acceptable. Selling copies is not. Filesharing?

You can make use of terms within this setting or system with impunity though.